It should be noted that the adjective “abradable” is commonly used to refer to a friable material capable of providing a certain degree of sealing against a surface moving relative to the material. This material can be constituted, formed or structured in a variety of ways. For example, an outer shell of an axial turbomachine compressor, fitted with a layer of this material ensures a rotating seal with the tips of the rotor blades of the compressor. This enables the gas-tightness of the whole to be maintained, despite any distortions, however slight, inherent in the turbomachine, particularly any deformation due to elongation of the blades resulting from centrifugal forces. By minimizing the clearance between the moving blades and the housing of the fluid stream, the effectiveness and efficiency of the turbojet is increased.
The use of composite materials, particularly those with organic matrices, is developing and growing, particularly for axial turbomachine compressors in aircraft engines. The use of such materials, especially as part of the outer shell brings new constraints.
Patent US2009/0277153 A1 discloses a stator of a gas turbine engine having a composite filling supporting a layer of abradable material. The role of this layer of abradable material is to line the inner surface of the stator housing of the engine in question to make frictional contact with the tips of the rotating blades. The inside of the housing is lined with a layer of composite cellular material designed to absorb potential impacts from ice or blade fragments in the event of a “fan blade-off.” The lining is coated with a layer of abradable material via an intermediate wall. This intermediate wall has a series of perforations with diameters greater than 1 mm. It is glued to the composite lining and is preferentially made of polyester reinforced with glass fibers. The layer of abradable material is fixed to the intermediate wall using an adhesive keying into the perforations. These perforations allow degassing to occur, particularly towards the cells of the composite lining, and enable a mechanical bond between the layer of abradable material and the intermediate wall.
In summary US Patent 2009/0277153A1 proposes a method of fixing an abradable material to a cellular support. The scope of this solution is therefore severely limited.
U.S. Pat. No. 4,329,308 discloses a metal ring with the edges having a cross section in the shape of a “U”, the hollow part being filled with an abradable material. This abradable material is formed by moulding it in the ring or is preformed and then placed in the ring. The abradable layer is then pressed against a toothed wheel in order to form cavities prior to being baked. As in the previous interpretation, the abradable material is made of a preformed polymer. It is stated in this patent that this ring can be inserted into a stator housing or form part of it. The method of fixing the ring to the stator is, however, not defined.
The solution proposed by this interpretation requires a particular method of producing and fixing the abradable layer. Furthermore, the proposed solution does not address the fixing of the ring to the housing.
U.S. Pat. No. 4,867,639 discloses an abradable layer including a honeycomb metallic structure and a ceramic component. The honeycomb-shaped structure is brazed directly onto the inner wall of the stator or, alternatively, onto a support attached to the stator wall. This abradable layer is more robust, and particularly more temperature-resistant than the abradable polymer of the above interpretations. However, as in previous interpretations, the abradable layer, due to its very nature, is put in place before fixing the support. In addition, the relatively substantial support is fixed into the housing with a mechanical joint, which is not compatible with a housing made of a composite material.
Beyond the interpretations of previous patents, high-performance abradable layers are nowadays deposited on metal shells using plasma spraying, especially in low pressure compressors. These abradable materials contain a metallic component that brings problems of retention when applied directly onto a composite support due to the significant difference in the coefficients of expansion of the metal and the composite and due to the fragile nature of the interface.